Current technologies provide the advantage that micromechanical and microelectronic components can be situated together on a chip (circuit element), for example on silicon chips. Piezoresistive pressure sensors are used in motor vehicles to measure the pressure in different media and pressure ranges. It is important, in particular, for pressure sensors of this type to be very accurate and very sturdy over their entire service lives and also economical to manufacture. According to the piezoresistive measurement principle, four piezoresistive electrical resistors are situated in a suitable location of a pressure measuring diaphragm or are mounted thereon.
The pressure measuring diaphragm is deformed by the pressure applied thereto; a deformation transferred from the pressure measuring diaphragm to the piezoresistive resistors resulting in a change in resistance of the resistors. In a Wheatstone bridge, which is provided as a measuring bridge, these three resistors are interconnected, the measuring bridge being able to be designed in such a way that it outputs an electrical signal as a bridge diagonal voltage which corresponds to the pressure of a medium to be measured. A deformation of the resistors results in a change in their resistance value (known as the piezoresistive effect), which causes the Wheatstone bridge circuit to become unbalanced. The Wheatstone bridge may be designed in such a way that the bridge diagonal voltage produces a signal which is proportional to the pressure applied so that an adjustable characteristic curve can be evaluated in a downstream evaluation circuit (evaluation electronics). Different designs of bridge circuits are known to those skilled in the art.
A technology known as silicon micromechanics makes it possible to efficiently integrate sensor functions such as the diaphragm, piezoelectric resistors, and evaluation circuit or evaluation electronics onto a single circuit chip (a single electronic circuit unit). This single-chip principle substantially reduces the cost of a pressure sensor of this type, due to the simple micromechanical and microelectronic structure. For example, there is no need for an additional circuit board, which would otherwise include the evaluation circuit; in addition, it is not necessary to establish contact between the evaluation circuit and circuit board over great distances.
A disadvantage of single-chip sensors of this type, which include both micromechanical and microelectronic components, is that they cannot be operated at high temperatures (usually not at temperatures above 150° C.), due to the microelectronic components situated thereon. Another disadvantage is the fact that aggressive media cannot be measured using these types of pressure sensors based on single-chip systems. The inability to detect pressures in the range above 10 bar is also unsuitable, since the single-chip systems are exposed to high mechanical forces in such a high pressure range.
A conventional pressure sensor, which is also designed for detecting pressure differences, is described in German Patent No. DE 39 28 542. The pressure sensor has a diaphragm on which sensor elements are mounted on the top and bottom, which are provided, for example, as interconnected piezoresistive resistors whose resistance changes due to diaphragm expansion. A diaphragm expansion of this type, and thus a change in the circuit resistance, is a measure of the pressure to be measured or the pressure difference to be measured.
The pressure difference sensor described in U.S. Pat. No. 4,895,026 can be used only in the case of low pressure differences, this sensor being disadvantageously provided with two different measuring diaphragms.
To be able to use conventional pressure sensors at high temperatures, in aggressive media, or to measure high pressures (above 10 bar), such pressure sensors are provided, according to the related art, with a metallic separating diaphragm on the back of which piezoresistive resistors are mounted. Thus, German Patent No. DE 37 03 685 describes a pressure sensor in which a resistant separating diaphragm is situated between the medium to be measured and the side of the measuring diaphragm facing the medium, the intermediate space being filled with an incompressible medium. This makes it possible to measure the pressure of even aggressive and/or corrosive media.
A considerable disadvantage of such pressure sensors is that they have a very complex design and, due to effects of the ambient conditions on the incompressible medium, which is used as an intermediate medium, an insufficient level of accuracy is achieved.
Piezoresistive resistors are conventionally mounted on the back of the separating diaphragm used as a protecting device. These resistors can be metallic or silicon resistors which are mounted by sputtering, bonding, or glazing. A structure of this type disadvantageously means that the evaluation circuit for evaluating the electrical signals supplied by the resistance sensors requires an additional circuit board.